Electrical resistance.



UNITED STATES PATENT ornron.

GIOVANNI MASCARINI AND ANGELO CONTARDI, OF MILAN, ITALY.

ELECTRICAL ansrs'mncn.

No Drawing.

To all whom it may concern Be it known that we, GIOVANNI MAs- CARINI and ANGELO CONTARDI, both subjects to 1 when its temperature varies from 14f It is known that triphase electric motors with wound rotors necessitate, in order that they may be started with a high torque, the insertion into the rotor circuit of a resistance-which must be subsequently gradually eliminated from the circuit in proportion as the speed increases. Whatever the construction of the rheostat may be, its gradual exclusion or elimination from. the circuit is effected both manually and automatically, mechanical actuating devices being only used in cases where they are absolutely necessary. These apparatus are complicated, delicate and expensive.

The use of rheostats made with iron sulfid renders it possible to construct a simple automatic rheostat especially in connection 'with rheostats Where the resistance diminishes as the temperature increases.

It isknown that during the starting period of a triphase motor having a wound rotor,v a certain quantity of energy proportional to the load is lost in the rheostat being converted into heat.

According to this invention the rheostat is formed by three elements of sulfid the resistance of which is determined by the resistance necessary for starting and the dimensionsof which are such that the gradual increase of the temperature due to the electric energy which in this rheostat is converted step by step into thermic energy produces a variation of resistance giving rise to a uniform increase in the speed of the motor. An automatic rheostat is thus obtained which subsequently permits of shortcircuiting the rotor. It is obvious that the dimensions Wlll vary according to the wind- Patented May 30, 1916.

Application filed January 3, 1914. Serial No. 810,177.

ing of the rotor and the load when starting. A rheostat of this kind can be readily arranged so as to rotate with the rotor by appropriately shaping its parts. The slip rings, brushes and conductors which connect the rotor to the stationary rheostat are therefore eliminated. A simple apparatus is only necessary to effect the short-circuiting so as to cut out the rheostat and to permit the latter to cool. Once this problem is solved it is easy to provide means for obtaining the automatic short-circuiting of the rotor.

The advantages obtained with such a rheostat are varied and are of great importance. 1. The starting operations of a motor of any power are reduced to the mere closing of a switch. 2. In the case of motors located in places difficult of access the short-circuiting of the rotor is possible by eliminating the resistance of the brush contacts and of the connecting circuit with the rheostat. 3. In the case of continuous supplies, the motor is restarted automatically when there are momentary interruptions in the feeding current. 4. In the case where the supply is effected from tank or reservoir pumps the motor maybe controlled by pressure or float apparatus. 5. Besides the aforesaid and other advantages, there is an economic advantage due to the simplicity of the apparatus and to the simplification in the installation of the motors.

The variations of the resistance and the temperature serve also for the application of sulfid rheostats for starting tr'iphase motors in short-circuit. The insertion in series with the stator winding, of a resistance varying from a maximum to a minimum, decreases the torque at the start and, considerably diminishes the differences of voltage in the circuits. In consequence thereof, when the motor is started under a reduced load, the automatic thermic rheostat is of a great advantage.

The variation of the temperature and therefore of the resistance being a function of the current which traverses the metal sulfid blade, and the dimensions of the latter being adapted to be selected in such a manner that for a predetermined intensity of current a certain amount of time should lapse before attaining a certain temperature, that is to say a certain resistance, it will be understood that it is easy to apply the properties of iron sulfid to time-controlled automatic apparatus. This susceptibility to variation in temperature and therefore in resistance, of a blade in proportion to the current assing therethrough, renders its application possible to the control of the voltage of electric generators.

What We claim and. desire to secure by Letters Patent of the United States is 1. A rheostat having a resistance element consisting of a homogeneous body of metallic 

